Conventionally well known are a TN (twisted nematic) liquid crystal display element and an STN (super-twisted nematic) liquid crystal element wherein nematic liquid crystal is utilized. But these liquid crystal display elements have a limited display capacity, since these liquid crystal elements' speed of electro-optical response is very low, in a millisecond order, which leads to defects such as disorder in the screen, deterioration in the contrast, or the like, when a high speed driving is attempted. Therefore, practical application of a liquid crystal display element using the ferroelectric or antiferroelectric liquid crystal has recently been considered, as liquid crystal display device of the next generation.
In 1975, in the light with the theory of the symmetry of the molecule, R. B. Meyer et al. presumed that optically active molecules which have a dipole moment in a direction perpendicular to the molecular major axis should exhibit ferroelectricity in a chiral smectic C phase (SmC* phase). They composited DOBAMBC (2-methylbutyl p-[p-(decyloxybenzylidene)-aminon]-cinnamate), and for the first time confirmed the ferroelectricity of liquid crystal (see R. B. Meyer, L. Liebert, L. Strzelecki, and P. Keller: Journal of Physics (Paris)36(1975)L.69).
The following description will explain a structure of liquid crystal in the SmC* phase which exhibits ferroelectricity. In the SmC* phase, the center of gravity positions of liquid crystal molecules in one layer are not in order, but major axes of the liquid crystal molecules (directors 102) are tilted at a uniform angle .theta. to a layer normal line z of layer planes 103 which serve as boundary between the smectic layers, as schematically illustrated by cones 101 in FIG. 10(a). Note that the directors 102 are tilted in directions slightly changing from one layer to another, thereby causing the crystal molecules to be aligned in a helical form. A pitch of the helix (helical pitch) is around 1 .mu.m, by far greater than a layer interval which is around 1 nm.
It has been confirmed that not only the ferroelectric liquid crystals but also some of antiferroelectric liquid crystals have the phase having the above-described molecular arrangement (see A. D. L. Chandani, T. Hagiwara, Y. Suzuki, Y. Ouchi, H. Takezoe, and A. Fukuda: Japan Journal of Applied Physics 27(1988)L729). Among the antiferroelectric liquid crystals, some have the SmC* phase depending on the optical purity, and some, like MMPOBC (4-(1-methylheptyloxycarbonyl)phenyl 4'-octyloxybiphenyl-4-carboxylate), have the SmC* phase even though they are rectus enantiomers or sinister enantiomers with the optical purity of 100 percent.
Clark and Lagerwall discovered that when the liquid crystal has a cell thickness of not more than around 1 .mu.m (around the helical pitch), the helical structure disappears and molecules 104 in each layer become in either of bi-stable states depending on an electric field applied thereto, as shown in FIG. 10(b), and proposed a surface stabilized ferroelectric liquid crystal (SSFLC) display element. This is disclosed in the Japanese Publication for Laid-open Patent Application No. 56-107216/1981 (Tokukaisho 56-107216), the specification of the U.S. Pat. No. 4367924, and others. Note that in FIG. 10(b), the electric field applied to the molecules 104 is directed from the reverse side to the front side of the sheet of paper having the figure so as to be perpendicular to the sheet of paper. In addition, all the electric dipole moments of the molecules 104 are directed in the direction of the applied electric field, as indicated in each molecule in FIG. 10(b).
The following description will explain an operation principle of the SSFLC element, referring to FIG. 11. As described, the SSFLC molecules 104 in a thin cell has either a stable state A or a stable state B in accordance with a direction of the electric field applied. Note that the electric field applied to the molecules 104 is, in the state A, directed from the front side to the reverse side of the sheet of paper having the figure so as to be perpendicular to the sheet of paper, while it is, in the state B, directed from the reverse side to the front side of the sheet of paper so as to be perpendicular to the sheet of paper.
Therefore, by providing the SSFLC cell between two polarizers orthogonal to each other so that, for example, in the state B the molecular major axes are parallel to one of the directions of the polarizers (direction 111 indicated by an arrow in the figure). With this arrangement, the SSFLC cell becomes bright in the state A since light is allowed to penetrate, while the SSFLC cell becomes dark in the state B since light is shut out. In other words, monochrome display can be realized by switching the directions of the electric field applied.
Since in the SSFLC a spontaneous polarization and the electric field applied directly and reciprocally act with each other, the SSFLC exhibits a high speed response to the electric field in a millisecond order, or a further quicker response, unlike in the case where switching operations are conducted with respect to a usual nematic liquid crystal by utilizing dielectric anisotropy. Furthermore, permanent application of a voltage is not required since the SSFLC has a feature that once being switched to either of the bi-stable states the maintains the state even after the electric field disappears, which is a so-called memory function.
Therefore, by utilizing the above-described characteristics of the SSFLC, i.e., the high speed response and the memory function, which make it possible to write data to be displayed to each scanning line at a high speed, it is possible to realize a simple matrix drive-type SSFLC display device having a large capacity. Its application to a wall TV set is also expected.
However, the conventional SSFLC display device has several problems. The serious problems to be solved in particular are to realize halftone display and to enhance shock resistance.
Though, strictly speaking, the ferroelectric liquid crystal is only capable of two-tone display due to the bi-stability of the liquid crystal molecules in the SmC* phase, arrangements enabling display in several tones have been proposed, which utilize the method of high speed modulation of the applied electric field, or the area division method. However, these conventional arrangements make the driving system or the panel manufacturing processes complicated and difficult, thereby raising the manufacturing costs, and hence causing the ferroelectric liquid crystal to be unsuitable for the practical application.
The enhancement of the shock resistance is also one of the problems which have not yet been solved, in view of the practical application of the arrangement wherein the ferroelectric liquid crystal is used. To be more specific, the SSFLC display device has defects that it is easily affected by pressure from outside, an electric shocks, or the like, and hence the alignment is easily disordered with the same. To solve this problem, an arrangement using spacer walls between substrates has been proposed, but various problems have arisen in the panel manufacturing processes, thereby resulting in that the practical application of the above arrangement has been unrealized.